Pre-baking laminates should only be performed:(1) to meet contractual agreements or (2) in response to an actual defect that can be traced to PWB water absortion. Besides the fact that a pre-bake is an enormous process "bottleneck," it also subjects the laminate to another temperature cycle and a little more degradation to the material. In order to remove moisture with any expedience, it is necessary to be close to, or to reach, the boiling point of water - 100C. Because this temperature is very close to the Tg of the laminate, there is very little margin for error, either in terms of the actual temperature or excessive time at temperature. The time near, at, or above 100C, will depend upon the thickness of the laminte - thicker laminates will require a longer time to bake. However, time periods will be on the order of 2 - 10 hours to remove moisture at temperatures close to 100C; the time stretches to tens of hours as the temperature is reduced.
But remember, after the board has been baked, it will begin to pick up moisture again. Therefore, it is necessary to control that time interval and/or storage condition between baking and subsequent stencil printing, placment, and finally reflow. If that time period is too long, then moisture will return to the laminate. At that point, not only was the pre-bake a waste-of-effort, but now, it must be made certain that the pre-heat stage of the assembly process will be adequate to provide the pre-bake function.
The above comments can be applied to rigid, rigid/flex, and flex product.